The present invention relates generally to optical filters, and more particularly to an optical, phase-only, spatial filter for filtering spatial frequencies from an output image without substantially reducing the intensity of that output.
Spatial filtering of a spectrum is routinely implemented to decrease error in the corresponding image. Such error is primarily encountered in an image which has been sampled, which results in a harmonic duplication of the original spectrum throughout the spatial frequency domain. Appropriate spatial filtering reduces this error.
Spatial filters for imaging systems currently exist in the form of a mask consisting of one or more apertures which must be very small to be effective and which therefore reduce the image intensity to an unacceptable level for most applications. A second method of applying spatial filtering uses Fourier transform algorithms on a computer to filter the digital image received from a detected array of image values. This second method is slow, does not exhibit real time performance, is expensive and yields a sampled image output limited in definition by the computer, camera and number of pixels on the monitor.
The invention described herein eliminates or substantially reduces in critical importance problems with prior art filters as just described by providing an optical, phase-only spatial filter comprising an optical element that is placed in a nonimage plane of an imaging system, the element consisting of a pattern of segments each producing an optical path length which differs from those produced by other segments as a result of differences in refractive index of the segments and/or in physical path lengths produced by the segments. The size of the segments, the optical path length required to be produced by each and the pattern which they form on the element are determined by the desired filtered optical transfer function for a given imaging system. Maximum filtration results when the optical path length of each segment differs from that of other segments by an amount greater than the coherence length of the light being imaged. Such differences smaller than the coherence length produce correspondingly smaller filtration effects. The invention has application to many areas of optical imaging, especially in reducing sampling effects in projected images.
It is therefore a principal object of the invention to provide an improved optical filter.
It is a further object of the invention to provide a filter for filtering spatial frequencies from an output image without substantially reducing the intensity of that image.
It is another object of the invention to provide an optical filter for reducing error in optically formed images.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.